Color aberrations in broadband imaging optics can be effectively corrected for by use of diffractive optical elements (DOE) such as kinoforms. Typically, the DOE groove width increases with wavelength range and is in the range of several ten to several hundreds of micrometers. Since the footprint diameter of a light bundle originating from a single object point at the diffractive surface is often in the range of millimeters, the number of grooves crossed by this light bundle can be small. In addition, the groove width varies and the grooves are curved. For DOE optimization and prediction of optical performance, optical design software is widely used being based on the ray trace formula, i. e. the law of refraction including DOEs. This ray trace formula relies on two assumptions. First, the footprint diameter of a light beam at the diffractive surface is assumed to be large compared to the groove width. Second, the local grating approximation is used saying that at the footprint area the groove width is constant and the grooves are straight lines. In realistic optical systems, these assumptions are often violated. Thus, the reliability of optical performance predictions such as MTF is in question. In the present paper, the authors re-examine the limits of the ray trace equation. The effect of a finite footprint diameter at the diffractive surface is investigated as well as variations of the groove width. Also, the Fraunhofer diffraction pattern of a light bundle after crossing a grating with a finite number of grooves is calculated.
Boundary integral equation methods (BIM) or simply integral methods (IM) in the context of optical design and simulation are rigorous electromagnetic methods solving Helmholtz or Maxwell equations on the boundary (surface or interface of the structures between two materials) for scattering or/and diffraction purposes.
This work is mainly restricted to integral methods for diffracting structures such as gratings, kinoforms, diffractive optical elements (DOEs), micro Fresnel lenses, computer generated holograms (CGHs), holographic or digital phase holograms, periodic lithographic structures, and the like. In most cases all of the mentioned structures have dimensions of thousands of wavelengths in diameter. Therefore, the basic methods necessary for the numerical treatment are locally applied electromagnetic grating diffraction algorithms.
Interestingly, integral methods belong to the first electromagnetic methods investigated for grating diffraction. The development started in the mid 1960ies for gratings with infinite conductivity and it was mainly due to the good convergence of the integral methods especially for TM polarization. The first integral equation methods (IEM) for finite conductivity were the methods by D. Maystre at Fresnel Institute in Marseille: in 1972/74 for dielectric, and metallic gratings, and later for multiprofile, and other types of gratings and for photonic crystals. Other methods such as differential and modal methods suffered from unstable behaviour and slow convergence compared to BIMs for metallic gratings in TM polarization from the beginning to the mid 1990ies.
The first BIM for gratings using a parametrization of the profile was developed at Karl-Weierstrass Institute in Berlin under a contract with Carl Zeiss Jena works in 1984–1986 by A. Pomp, J. Creutziger, and the author. Due to the parametrization, this method was able to deal with any kind of surface grating from the beginning: whether profiles with edges, overhanging non-functional profiles, very deep ones, very large ones compared to wavelength, or simple smooth profiles. This integral method with either trigonometric or spline collocation, iterative solver with O(N2) complexity, named IESMP, was significantly improved by an efficient mesh refinement, matrix preconditioning, Ewald summation method, and an exponentially convergent quadrature in 2006 by G. Schmidt and A. Rathsfeld from Weierstrass-Institute (WIAS) Berlin.
The so-called modified integral method (MIM) is a modification of the IEM of D. Maystre and has been introduced by L. Goray in 1995. It has been improved for weak convergence problems in 2001 and it was the only commercial available integral method for a long time, known as PCGRATE.
All referenced integral methods so far are for in-plane diffraction only, no conical diffraction was possible. The first integral method for gratings in conical mounting was developed and proven under very weak conditions by G. Schmidt (WIAS) in 2010. It works for separated interfaces and for inclusions as well as for interpenetrating interfaces and for a large number of thin and thick layers in the same stable way. This very fast method has then been implemented for parallel processing under Unix and Windows operating systems.
This work gives an overview over the most important BIMs for grating diffraction. It starts by presenting the historical evolution of the methods, highlights their advantages and differences, and gives insight into new approaches and their achievements. It addresses future open challenges at the end.
We report on a fast and accurate shape metrology for nanoscale structures by analyzing the scattering pattern
of visible light. The technique is based on model-based scatterometry which is inverse measurement technique
comparing measured scattering data with numerical simulations of the scattering process using a physical model
of the structures. We demonstrate the concept for an array of silicon nanopillars that are arranged in a twodimensional
lattice and show that the proposed methodology provides a fast and reliable determination of the
pillar dimensions with nanometer precision. Since the technique works contact-free and is applicable to large
area samples, it can be readily implemented in an industrial environment for inline metrology applications.
Finite element methods (FEM) for the rigorous electromagnetic solution of Maxwell's equations are known to be
very accurate. They possess a high convergence rate for the determination of near field and far field quantities
of scattering and diffraction processes of light with structures having feature sizes in the range of the light
wavelength. We are using FEM software for 3D scatterometric diffraction calculations allowing the application
of a brilliant and extremely fast solution method: the reduced basis method (RBM). The RBM constructs a
reduced model of the scattering problem from precalculated snapshot solutions, guided self-adaptively by an error
estimator. Using RBM, we achieve an efficiency accuracy of about 10-4 compared to the direct problem with
only 35 precalculated snapshots being the reduced basis dimension. This speeds up the calculation of diffraction
amplitudes by a factor of about 1000 compared to the conventional solution of Maxwell's equations by FEM.
This allows us to reconstruct the three geometrical parameters of our phase grating from "measured" scattering
data in a 3D parameter manifold online in a minute having the full FEM accuracy available. Additionally, also
a sensitivity analysis or the choice of robust measuring strategies, for example, can be done online in a few
minutes.
Diffractive optical elements (DOE) are well-suited for the correction of longitudinal and transverse chromatic
aberrations in broadband optical systems such as photographic lenses, HMDs or infrared lenses. Unfortunately,
the diffraction efficiency η for the working order of DOEs is often clearly below 100% which causes stray light
from unwanted diffraction orders and prevents a more frequent use of DOEs in practice. The main reasons for
the decrease of diffraction efficiency η are spectral bandwidth, variation of the incidence angle of impinging light
and manufacturing errors of the DOE. In the present paper, several scalar approximations for the diffraction
efficiency η as a function of these parameters are compared with rigorous electromagnetic calculations. The
validity of these formulae are shown to extend over a surprisingly large range of parameters.
Light transmission through circular subwavelength apertures in metallic films with surrounding nanostructures
is investigated numerically. Numerical results are obtained with a frequency-domain finite-element method.
Convergence of the obtained observables to very low levels of numerical error is demonstrated. Very good
agreement to experimental results from the literature is reached, and the utility of the method is demonstrated
in the investigation of the influence of geometrical parameters on enhanced transmission through the apertures.
Depending on the specific application of a diffractive optical
element (DOE), its polarization impact on the optical system must be
taken into account. This may be necessary in imaging as well as in
illumination optics, e. g., in miniaturized integrated optics or in
high-resolution photolithographic projection systems. Sometimes, polarization effects are unwanted and therefore an exact characterization of their influences is necessary; in other cases a high polarization effect is the goal. It is well known how to calculate the point spread function (PSF) of a single diffractive micro-Fresnel lens. To do the same for a complete optical system with source, lenses, coatings, mirrors, gratings and diffractive elements, a 3D electrical field propagation along the geometric optical path is introduced into the ray-trace based optical systems design software in order to incorporate the entire electromagnetic polarization effects from the source to the image plane. Our software also considers the complex diffraction amplitudes including polarization effects from DOEs provided by rigorous electromagnetic methods. Together with a plane wave decomposition and with the local linear grating assumption, we are able to rigorously investigate the impact of e. g. polarization effects on the PSF of the whole optical system. Using this approach we analyze a hybrid diffractive-refractive microscope objective for mask inspection systems at 193 nm. Additionally we investigate focal properties of a sample diffractive blue laser disc pickup system.
The radiation emitted from an EUV source is collected and focused by a suitable collector system. A reflective blazed grating is used in -1st diffraction order to select a definite spectral band around 13.5 nm wavelength from the broad-band emission spectrum of the source. The effective grating area is segmented into a set of different plane gratings, mounted on a common base plate. In order to focus the light from the collector system, the grating segments are tilted and form a best-fit polygon surface. A specific groove density variation on the grating segments significantly improves the imaging performance. In this paper, we report on design, fabrication and testing of the grating system.
For the design and characterization of synchrotron monochromators in the XUV region it is advantageous to know the performance of the mirrors and gratings as exact as possible within the wavelength range of operation to get performance results under realistic conditions. For that reason, monochromator gratings for dipole radiation as well
as for undulator radiation at BESSY II are investigated in their wavelength range of operation by two different methods of electromagnetic grating theory. The IESMP, developed at BIFO, is a special boundary integral method, while the other, LUMNAB, developed by M. Nevière and available at BESSY, is based on the differential equation method. Both methods are compared with measurements on a plane grating in fixfocus condition for an energy range
from 150 eV to 1000 eV. The observed differences between both methods and between the numerical results and measurement are discussed. For a grating monochromator in a variable included angle scheme efficiencies from the IESMP calculations for the first and higher orders are compared with scalar theory. Because of significant differences in the predictions of both methods this has to be verified by measurement.
In this paper four radically different methods for solving scattering of light off of mainly lameller grating type structures are compared. The methods are: (1) the analytic waveguide method (Metrologia), (2) the integral equation system with parametrization and sampling along the profile (IESMP) developed at BIFO, (3) the integral equation method with discretization along the x-axis (PC-Grate 2), and (4) the rigorous coupled-wave analysis (RCWA). The results show that the analytic waveguide method and the integral equation method with parametrization (IESMP) agree with extremely high accuracy in all cases studied for both polarization states TE and TM. For the special case of near field slit simulation the same high agreement is found between Metrologia and a FEM method. It is also found that the agreement of RCWA and PC-Grate 2 with either of the other two methods is only fair, particularly for TM mode on gratings made of good conductors. Using an improvement, the RCWA results become significantly better and correspond with the results of IESMP and Metrologia.
Rigorous models for simulations in optical lithography have become increasingly important as the feature sizes being formed are ever closer to the resolution limit. The novel approach of integral equation system method with sampling along the profile (IESMP) is investigated and compared with other rigorous approaches such as the rigorous coupled-wave analysis (RCWA) and the time-domain finite-difference method. The IESMP, essentially based on a parameterization of the topography being simulated and able to treat all kinds of structures, including those with vertical boundaries and with overhanging parts, is a very flexible and exact method. So, it could serve as a gauge for the comparison of different algorithms and codes. As it is shown, IESMP and RCWA calculations for TM polarization already differ for shallow metal structures.
The trend of the increase of the output energy is obviously as good for UV lasers as for IR lasers. This requires radiation resistant elements, for instance lenses, mirrors, or gratings. Monochromatic radiation can be focused by diffractive optical elements to a diffraction limited focal spot with nearly 100% efficiency. Therefore, diffractive optical elements are well suited for application to industrial laser cutting and welding technology. However, usual holographic gratings are destroyed by an irradiation of a few watts per cm2. To overcome this difficulty electroforming and etching techniques have been developed. We report on the production, calculation, and measurements of efficiency of gratings in copper with grating constants between 0, 5, and 200 micrometers .
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